    [Patent document 1] JP-2003-81014 A    [Patent document 2] JP-2005-35542 A
There is conventionally known a technology to display a captured image of a circumference outside of a vehicle, thereby eliminating a dead angle of the vehicle to help prevent a collision. For example, Patent document 1 discloses a technology to capture an image to cover all angles of a circumferential area at the rear of the vehicle using a rear monitor camera, thereby enabling the recognition of the dead angle which cannot be confirmed by a vehicle mirror.
In addition, there is recently known a technology to use a bird's-eye view image as an effective means to make it easy to understand positional relation between a vehicle and an obstacle, thereby helping prevent an accidental contact. For example, Patent document 2 discloses a technology to display a bird's-eye view image which illustrates an omnidirectional circumference state outside of the vehicle viewed from a viewpoint above the vehicle, thereby making it easy to understand positional relation between the vehicle and the obstacle.
In order to display a bird's-eye view image which illustrates a circumference state of all the directions or areas surrounding the vehicle, a general technology is used as follows. That is, several cameras are installed in the vehicle so as to capture images of several circumferential areas. Such captured images by the cameras are inputted into an electronic control unit (ECU) to undergo a bird's-eye view transformation; the cameras' images having undergone the bird's-eye view transformation are combined to generate a single bird's-eye view composite image.
It is noted that the longer a cable connecting the camera with the ECU is, the more an analog signal from the camera to the ECU attenuates. Thus, a video signal is attenuated more with respect to a camera having a longer cable connecting with the ECU. As a result, an image originating from a camera having the longer cable is faded more in color definition. Further, suppose the case that the several images originating from the cameras having the cables with much mutually different lengths are combined to thereby generate or synthesize a single composite image. In such a case, the originating images are different from each other in the color definition; thus, an appearance of the synthesized single composite image is degraded to reduce a commodity value, posing a disadvantage.
It is noted that such a disadvantage arises more significantly as the size of the vehicle becomes larger to cause the installation positions of the cameras to be separated farther from each other. Therefore, in particular, such a disadvantage arises remarkably with respect to a large-sized vehicle such as a track and a bus.
In addition, a countermeasure to prevent the occurrence of the disadvantage may be adopted which applies A/D conversion to the images captured by the cameras and then transmits resultant digital video signals of the several cameras to the ECU via the corresponding signal cables. In this regard, however, such a countermeasure requires each of the cameras or camera units to have a function to execute the A/D conversion. This poses the cost increase and another disadvantage to make it difficult to reduce the size of each camera or camera unit.